Unable to locate font code in CSS to resize (14 posts)

The default font size specified is really too small. I would like to increase this size but I am not a coder. Despite looking at many help sites, I am still unable to locate the code in the CSS file that specifies the code - especially to control the font size for the excerpts on the top/home page and the body content on the single post page.

i tried that but gave up and resorted to just defining the paragraph in pixels, which i realize does not work in ie.

even when i tried to define the font size in em's in the css it does not seem to control the pages, so i figured it's getting overridden in the various templates used to build each of the relevant pages

i can really tell that i am out of my element here - thank you for trying to help me.
i have tried to set the parameters at the end as your recommended but there are still inconsistencies - a the header for example on a single post view is larger than the body copy even though they are both set to 1em

this is the current css code - if you have the patience to help me clean it up i would be most thankful

yes, i was just trying to see these new tags would bring consistency but they don't as the post title for example on single post page is still bigger than the body/paragraph text - by any chance do you know how to control the font size for the excerpt that appears on the top page?